The radiologic progression of spondylolisthesis during a long-term follow-up was studied in 272 children and adolescents. There were 134 girls and 138 boys. The mean age at the first visit was 14.3 years (girls, 13.8 years; boys, 14.9 years). The radiologic follow-up time was 14.8 years on average (range, 5–32). The operation was done in 190 patients younger than 20 years of age. Fusionin situ, using a posterior or posterolateral technique, had no statistically significant effect on progression. Surgically treated patients did not differ from conservatively treated patients. Ninety percent of the slip, on average, had already occurred at the time of the first radiologic examination compared with the final amount of slip. More than 10% progression occurred in 62 patients, mainly within the first year postoperatively or after the first examination. Progression of the lumbosacral kyphosis and sinking of the vertebral body was noted in severe slips. Although female gender and dysplasia (spina bifida) at the lumbosacral junction were more frequent in severe slips, they statistically had no value in predicting progression. A wedge form of L5 or sacral rounding also had no prognostic value. These were secondary to the slip and expressed it but did not predict it. The only radiologic variable with predictive value of progression was the percentage amount of the primary slip. In age groups corresponding to the growth spurt in early puberty (girls, 9–12 years; boys, 11–14 years), there was a tendency to progress.